如何在TypeScript中声明变量具有数组元素的类型？

Question

In TypeScript, it is possible to declare that, eg, var b has the same type as var s :

var s = "hello";
var b: typeof s;

Is there a simple way to declare that var b has the same type as the elements in an array? Here, var b should still be a string:

var c = ["hello", "world"];
var b: typeof(c[0]);

An ugly workaround (which runs even if c is null) is:

var c = ["hello", "world"];
var a = (<T>(x: T[]): T => null)(c);
var b: typeof a;

Is there a better way?

Answer 1

Is there a better way?

No. There is no dedicated syntax for it. In the absence of a dedicated syntax you fundamentally need something that takes T[] and gives you T . You in-line function is a decent example of this generic usage.

Answer 2

Couldn't find anything in the language specification about this, and to me this seems like something too trivial to be implemented; at least at the current stage that TypeScript is in.

However, a shorter/simpler workaround could be:

var a = 0 && c[0];
var b: typeof a;

Should work even if c is null or undefined .
Much less of a performance impact as well.